Field of the Invention
The present invention relates to a driving force transmission mechanism using a timing belt for an internal combustion engine, and particularly relates to a driving force transmission mechanism capable of decreasing a local load working on the timing belt during engagement of the timing belt with a timing pulley.
The engagement portion of a timing belt and a timing pulley, used for a driving force transmission mechanism of an internal combustion engine, is constituted generally as shown in FIG. 17. Face width W2 of the teeth of timing pulley 2 is set larger than face width W1 of the teeth of timing belt 1 to prevent timing belt 1 from disengaging with timing pulley 2. Therefore, teeth 3 of timing belt 1 engage with the teeth of timing pulley 2 over the entire length of teeth 3, and a local load is liable to add at a corner 4 of the dedendum of tooth 3, shown in FIG. 18. The corner 4 of the dedendum of tooth 3 is positioned at a side surface of timing belt 1, and is the severest portion with respect to strength in timing belt 1. Particularly, when torsional vibration or other vibration occurs in timing belt 1, a load is liable to concentrate to corner 4 of the dedendum of tooth 3. Ordinarily, a covering layer (for example, canvas 5 constructed of synthetic fiber, cotton, etc.) excellent in accordance with abrasion resistance is provided on the surface having the teeth of timing belt 1, and thereby the durability of the surface having teeth is increased.
However, timing belt 1 is usually manufactured by cutting or splitting a belt, having a ring-like shape and having a large size width, into members of ring-like shape. Therefore, both width end surfaces of timing belt 1 have cut surfaces of canvas 5 on account of the above manufacturing, and since fibers constituting canvas 5 are also cut at the width end surfaces, these portions become weak in strength.
In such a timing belt 1, if there occurs torsional vibration or waving of timing belt 1 or inclination or bending vibration of timing pulley 2, a load concentrates at corner 4 of the dedendum of teeth 3 on the side surface of timing belt 1 as aforementioned. Thus, there is the possibility that fibers of canvas 5 may be broken at the portion. The breakage of fibers of canvas 5 would eventually cause breaking of canvas 5 itself, cracking of teeth 3 at the dedendum or breaking off of teeth 3 in timing belt 1.